1. Field of the Invention
This invention relates to color cathode ray picture tubes, and is addressed specifically to an improved tension shadow mask and its support structure as used in cathode ray tubes having a flat faceplate and a striped screen.
A shadow mask is a part of the cathode ray tube (CRT) front assembly, and is located in close adjacency to the screen. The shadow mask is an apertured metal component that acts as a color-selection electrode, or "parallax barrier," which ensures that each of the three beams generated by the electron gun located in the neck of the tube lands only on assigned phosphor targets on the screen.
The mask is positioned in a predetermined relationship with the screen by two or more mask support structures affixed to the inner surface of the faceplate, and which are referred to hereafter as "rails." The apertured foil that comprises the mask is stretched over the rails and welded to a mask-receiving surface on the rails. In tubes having a flat faceplate and a flat tension mask ("FTM" tubes), the mask is spaced a predetermined, constant distance from the screen by the rails, a dimension known as the Q-spacing.
The type of shadow mask of interest in the present disclosure is categorized as the strip mask, depicted schematically in FIG. 1. Strip mask 1 consists of thin strips 2 of metal which extend the full height of the imaging screen, and which are separated by slits 3. The strips 2 are attached to skirts 4 and 5 which in turn are affixed to the mask-receiving surface of two rails located on opposed sides of the screen (not indicated). Stripes of light-emitting phosphor corresponding to the slits 3 are printed on the screen. This type of screen, referred to in the present disclosure as a striped screen, is also known as a line screen.
The strip mask 1 shown by FIG. 1 is formed from a metallic foil which may have a thickness of 0.0003 inch to 0.002 inch, with the thickness dependent upon the size and application of the CRT. Such thin foils are basically non-self-supporting so they must be installed in a highly tensed state on the rails. By way of example, the tension of a foil mask for a 14-inch (diagonal measure) CRT is about 30 kpsi.
As with all types of shadow masks, the problem of degrouping is present in strip masks. Degrouping is a condition in which the adjacent trios of electron beams become increasingly crowded as a function of distance from the center of the screen. Degrouping has the undesired effects of reducing useful illumination at the sides and corners of the screen, and the dilution of the color image.
Degrouping is caused primarily by (1) the movement of the deflection center of the electron beams toward the screen with increase in the deflection angle of the beams as they move toward the sides of the screen, and (2) the influence of the yoke.
Regarding (1), the first cause: please refer to FIG. 2A, which depicts the paths of undeflected and deflected trios of electron beams. An electron gun (not indicated) of a CRT 6 emits three beams 7, 8 and 9. The beams pass through a shadow mask 12 mounted on rail(s) 13 affixed to faceplate 14. Each of the beams 7, 8 and 9 impinges upon a specific color in the trios of red, green, and blue-light emitting phosphor stripes deposited on screen 11. If the color phosphor stripes are printed on the panel to correspond with the landing areas of the respective beams which excite them, adjacent trios of phosphor stripes become crowded during the printing process.
The status of representative trios of electron beams at the center and the corners of the screen 11 are depicted in FIG. 2B, with the color that each of the three beams excites--whether red, green or blue--indicated symbolically. It is to be understood that the trios depicted are segments of electron beams that extend the full height of the screen 11. The intra-trio grouping (i.e., grouping of beams within trios) depicted in FIG. 2B is typical of a flat tension mask supported by rails having a constant Q-spacing. When the beams 7, 8 and 9 fall on the patterns of stripes at or near the center 15 of screen 11, they deflect at deflection center 10A. The trios of electron beams at center 15 are said to be "grouped"; that is, the intra-trio beam spacing is less than the inter-trio beam spacing (spacing between trios), a grouping illustrated by group 16 in FIG. 2B.
However, when beams 7, 8 and 9 are deflected toward the corners of the screen 11, for example at corner location 17, the trios of beams, represented by trio 16A, become "degrouped," or spaced apart as indicated by FIG. 2B. The intra-trio degrouping is a function of the deflection angle of the beams 7, 8 and 9: as the beams move toward the sides or corners of the screen 11, the effective deflection center, indicated by beam deflection center 10B, moves toward the screen 11. As a result, the trios become degrouped. This condition is shown by trio 16A, in which the red and blue outer beams crowd the respective blue and red outer beams of the adjacent trios.
The distribution of trio grouping and degrouping across a screen is indicated by FIG. 2C, in which the status of the trios is indicated by the symbols "g" for grouped, "d" for degrouped, and "sd" for severely degrouped. As indicated, degrouping is most severe in the corners of the screen.
With regard to item (2)--the influence of the yoke as a cause of degrouping, it is difficult to design a yoke which does not cause at least some degrouping. Some yoke designs will cause only minor degrouping, while in other designs, the degrouping will be severe. Effective yoke design is a process of many trade-offs, of which the extent of degrouping is only one.
Degrouping can be alleviated by printing the screen so that the phosphor stripes are narrower at the sides and corners of the screen in order to avoid loss of color purity. The penalty however is a reduction in brightness.
Degrouping can also be alleviated by grading the shadow mask; that is, by varying the pitch, or spacing of the strips, as a function of the distance from the center of the mask to its periphery. In a strip-type shadow mask for striped screens, the distance between the strips is increased about eight percent from the center of the mask to the sides. Such grading, however, undesirably coarsens the pattern of phosphor stripes at the sides of the screen. Further, the need to grade the mask complicates its manufacture.
Another remedy, a partial one, depends on a compromise in establishing the Q-distance. The extent of intra-trio degrouping depends on the spacing of the mask in relation to the screen. If the Q-distance is selected to avoid trio grouping at the center of the screen, adjacent trios of beams in the corners of the screen become unacceptably crowded; i.e., degrouped. By reducing the Q-distance, a slight but tolerable grouping will occur at the center of the screen, while the degrouping at the corners is alleviated.
2. Discussion of Related Art
A form of the strip mask, the subject of U.S. Pat. No. 3,638,063 to Tachikawa et al, is shown by FIG. 3. It is an etched mask consisting of a parallel array of narrow strips the ends of which are attached to a curved spring frame which holds the strips under tension, forming a sector of a cylindrical surface that is in parallel with a curved faceplate. The curvature of the spring frame is designed to conform to the curvature of the face panel with which it is associated. Disadvantages inherent in a mask assembly of this type include its bulk and weight and the complexity of its manufacture. Also, the strips tend to vibrate independently. As indicated in FIG. 3, the latter deficiency is remedied in Tachikawa et al by the stretching of one or more fine wires or fibers over the cylindrical surface, which serve to dampen vibration by contact with the strips.
Another form of strip mask, one that uses wires as the shadowing elements, is disclosed in U.S. Pat. No. 2,842,696 to Fischer-Colbrie.